Reliable biomarkers to assess the activity level in Mycobacterium tuberculosis infection (Mtb) are urgently needed for targeted interventions towards the prevention of disease. This need is particularly high in the setting of HIV co-infection which is a major risk factor for active tuberculosis (TB) TB, the leading cause of death among people with HIV. Although latent Mtb infection (LTBI) and TB are commonly seen as binary states, reactivation and disease are preceded by a continuum of increasing infection activity within LTBI. In addition to being a strong risk factor for reactivation, HIV is also a ris factor for progression to TB due to exogeneous new or reinfection which is often rapidly progressive. The proposed studies seek to identify host protein and antibody (Ab) responses as correlates for Mtb infection activity in asymptomatic people living with HIV (PLHIV). Identificatio of such biomarkers could lead to the development of new diagnostics to predict the risk for reactivation in PLHIV, which could help optimizing the timing of preventive therapy initiation and may increase its effectiveness. Using liquid chromatography and mass spectrometry (LCMS), we have identified host proteins that are significantly differentially expressed in the sera of HIV individuals with TB compared to those with quiescent LTBI, or other respiratory diseases. Utilizing our novel Mtb protein microarray based on a unique nucleic acid programmable protein array (NAPPA) format that allows screening of sera for Abs to the entire Mtb proteome we have identified ~220 protein targets that are recognized by HIV+ TB patients but not those with quiescent LTBI. These preliminary data provide us with already identified selections of potential host biomarkers for further evaluation in our proposed studies. Our overarching hypothesis is that host protein and Ab profiles can constitute a biomarker for increasing Mtb infection activity and predict the risk for development of TB in PLHIV. Using novel innovative techniques, we propose to study prospectively collected stored samples from US and South African HIV+ cohort subjects up to two years pre and one year post development of TB (n=110), and compare them to those who have not developed TB. With these samples, we will address the following aims: Aim 1. Determine host protein biomarkers for increasing Mtb infection activity in PLHIV; Aim 2. Characterize Ab profiles associated with development of TB in PLHIV; and Aim 3. Develop prediction models for risk of TB in PLHIV. At the completion of the proposed studies we anticipate having identified single and/or multi-platform biomarkers as correlates for Mtb infection activity, and developed prototypes of targeted detection assays for further validation in large multi-center prospective studies.